1. Field of the Invention
The present invention relates to an optical device having an optical waveguide type diffraction grating device, an optical module including this optical device, an optical amplifier including this optical device, and an optical transmission system including at least either of these optical module and optical amplifier.
2. Related Background Art
An optical waveguide type diffraction grating device is an optical device in which an optical waveguide (e.g., an optical fiber) is formed with a grating based on refractive index modulation over a predetermined range along the longitudinal direction of the optical waveguide, and can selectively reflect or attenuate light of a specific wavelength from the light propagating through the optical waveguide by way of the grating. Here, the “grating” simply referred to in this specification includes: Bragg gratings for Bragg-reflecting light of a specific wavelength having propagated through an optical waveguide to transmit thus reflected light in the opposite direction; tilted Bragg gratings having a tilted refractive index modulation for Bragg-reflecting light of a specific wavelength having propagated through the optical waveguide and causing a loss thereto; and long-period gratings for converting core-mode light of a specific wavelength having propagated through an optical waveguide into cladding-mode light and causing a loss thereto.
An optical waveguide type diffraction grating device having a Bragg grating is applied as a constituent of an optical module for reflecting light of a specific wavelength to multiplex or demultiplex the light or change a path of the light (e.g., optical multiplexer, optical demultiplexer, optical ADM (Add-Drop Multiplexer), or optical XC (Cross Connect)). An optical waveguide type diffraction grating device having a tilted Bragg grating or long-period grating acts as a loss filter for causing a loss in light of a predetermined wavelength, and is applied as a constituent to a gain equalizer for equalizing gain in an optical amplifier, for example. The optical module or optical amplifier is widely used in a wavelength division multiplexing (WDM) communication system for transmitting multiplexed signal light of a plurality of channels, and the like.
Meanwhile, an optical waveguide type diffraction grating device which can dynamically adjust its reflection spectrum is disclosed in Japanese Patent Application Laid-Open No. 2000-98146 (First Reference). The optical waveguide type diffraction grating device disclosed in this First Reference is one in which an optical fiber is formed with a Bragg grating, the optical fiber being longitudinally expanded by a mechanical or magnetic force (with a solenoid, for example), so as to extend the grating period, thereby shifting the reflection wavelength to the longer wavelength side. Optical modules using such an optical waveguide type diffraction grating device can dynamically adjust multiplexing and demultiplexing of light or changing of the optical path.
On the other hand, an optical waveguide type diffraction grating device can selectively reflect or attenuate light of a specific wavelength from light propagating through an optical waveguide by way of a grating. Particularly, the optical waveguide type diffraction grating device having a chirped Bragg grating whose refractive index modulation grating period changes along the longitudinal direction can reflect light of a wavelength satisfying a Bragg condition at each position in the longitudinal direction of the grating, and thus can be used as a constituent of a dispersion-adjusting module for adjusting the chromatic dispersion of light in a fixed wavelength region. The dispersion-adjusting module is provided in a repeater or the like, or just in front of a receiver in an optical transmission system which transmits signal light, and can compensate for the chromatic dispersion of an optical fiber transmission line.
In the dispersion-adjusting module including an optical waveguide type diffraction grating device having a chirped Bragg grating, its dispersion characteristic is preferably adjustable. For example, each of dispersion-adjusting modules mass-produced with a predetermined spec can be installed in a repeater or the like, or just in front of a receiver, so that the dispersion characteristic of the dispersion-adjusting module can be adjusted according to the dispersion characteristic of the optical fiber transmission line to be subjected to dispersion compensation, whereby dispersion-adjusting modules can be made inexpensively. Also, when the dispersion characteristic of the optical fiber transmission line to be subjected to dispersion compensation varies due to a temperature change, the dispersion characteristic of the dispersion-adjusting module can be adjusted according to the variation, whereby the chromatic dispersion of the optical fiber transmission line can always be compensated for by the dispersion-adjusting module favorably.
As an optical member including an optical waveguide type diffraction grating device which is applicable to such a dispersion-adjusting module having a variable dispersion characteristic, those disclosed in Japanese Patent Application Laid-Open No. 2000-235170 (Second Reference) and a literature,—M. M. Ohn, et al., “Dispersion variable fibre Bragg grating using a piezoelectric stack”, Electronics Letters, Vol. 32, No. 21 (1996) (Third Reference)—have been known.
A technique disclosed in the above-mentioned Second Reference has an optical fiber that is an optical waveguide, and is formed with a grating over a predetermined range along the longitudinal direction thereof, whereas a plurality of microheaters are disposed in contact with the optical fiber in the predetermined range. These plurality of microheaters form a temperature distribution in the above predetermined range of the optical fiber, which adjusts the effective refractive index of the grating at each position, thereby regulating the dispersion characteristic of the grating upon reflection of light.
The above-mentioned Third Reference disclose a structure in which a grating is formed over a predetermined range along the longitudinal direction of an optical fiber which is an optical waveguide, and a plurality of piezoelectric devices are disposed in contact with the optical fiber in the predetermined range. These plurality of piezoelectric devices form a stress distribution in the predetermined range of the optical fiber, which adjusts the grating spacing of the grating at each position, thereby regulating the dispersion characteristic of the grating upon reflection of light.